An optical code division multiplexing method by which same propagation media and same optical frequency bands can be shared simultaneously by a plurality of signals by identifying by codes has been investigated as optical communication used in the future. In particular, optical code division multiplexing coded by amplitude, phase, and frequency in optical frequency or wavelength region, where blocking of an interference light by improper connection is allowed, is promising.
However, with optical code division multiplexing in the optical frequency region or wavelength region, even with bipolar method or pseudo-bipolar method which enables suppression of multiple access interference, degradation of sensitivity due to beat noise between coded lights of a plurality of codes sharing a medium and optical frequency band and shot noise of coded light of a plurality of codes sharing a medium and frequency band can not be ignored, and there exists a problem that a limitation is imposed to the number of multiplexed codes (see, for example, Non-patent Document 1). The following description explains this problem.
FIG. 1 shows one example of configuration of pseudo-bipolar OCDM-PON which is PON (Passive Optical Network) subjected to optical code division multiplexing (OCDM) by connecting ONUs (Optical Network Units) which are a plurality of user side equipments to an OLT (Optical Line Terminal), that is single station side device, via an optical coupler/splitter 112 and a single optical fiber.
In ONU101-1, modulated light which is of light from a light source 121 and is modulated by a modulator 122 according to user's transmission data is coded by a coder 123 and is output. The coder 123 follows a specific code assigned for every ONU-101-1, 101-2 to 101-n. At OLT 111, coded light being coded by a code different depending on every ONU, from a plurality of ONU-101-1, 101-2 to 101-n is decoded, and is detected by differential detectors 132a, 132b. 
Here, as for the code used in coding in the coder 123, a code in which multiple access interference is suppressed by decoding by a receiver side decoder 131 and differential detection by the differential detectors 132a, 132b at receiver state is used. In ON/OFF light intensity modulation, for such code, Hadamard code or cyclic bit-shifted M-sequence code is mentioned.
When such code is used, optical frequency chips that assigned a value of “1” by the code of receiving object is mostly input to one side of the differential detectors 132a, 132b, and is not input to other side. In this case, optical frequency chips that assigned a value of “1” by the code of other than receiving object, is input to both sides of the differential detectors 132a, 132b with nearly uniform intensity. For this reason, the optical frequency chips constituting the code other than the receiving object are balanced but by differential detection, multiple access interference is cancelled ideally.
In the pseudo-bipolar OCDM-PON shown in FIG. 1, coded light Ei of code i, suppression ratio αi of multiple access interference of code i to the decoder 131 corresponding to code p, and noise variance σ2 after detection using the decoder 131 corresponding to the code p are expressed by the following equations, respectively.
                              E          i                =                                            ∑              m                        M                    ⁢                                    E              im                        ⁢                          cos              ⁡                              (                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      im                                        ⁢                    t                                    +                                      ϕ                    im                                                  )                                                                        Equation        ⁢                                  ⁢                  (          1          )                                                  α          i                =                              ∑            m            M                    ⁢                                    (                                                C                  pm                                -                                  C                  pm                  ′                                            )                        ⁢                                          E                im                2                            /                                                ∑                  m                  M                                ⁢                                                      (                                                                  C                        pm                                            -                                              C                        pm                        ′                                                              )                                    ⁢                                      E                    pm                    2                                                                                                          Equation        ⁢                                  ⁢                  (          2          )                                                              σ            2                    =                                                    a                1                            +                              a                2                            +                              a                3                            +                              a                4                            +                              a                5                                      ≈                                          2                ⁢                e                ⁢                                                                  ⁢                BR                ⁢                                                      ∑                    m                    M                                    ⁢                                                            (                                                                        C                          pm                                                +                                                  C                          pm                          ′                                                                    )                                        ⁢                                                                  D                        p                                            ⁡                                              (                        t                        )                                                              ⁢                                          E                      pm                      2                                                                                  +                              eBR                ⁢                                                      ∑                                          i                      ;                                              i                        ≠                        p                                                              K                                    ⁢                                                            ∑                      m                      M                                        ⁢                                                                  (                                                                              C                            pm                                                    +                                                      C                            pm                            ′                                                                          )                                            ⁢                                              E                        im                        2                                                                                                        +                                                1                  2                                ⁢                                  R                  2                                ⁢                                                      ∑                                          i                      ;                                              i                        ≠                        p                                                              K                                    ⁢                                                            ∑                      m                      M                                        ⁢                                                                  (                                                                              C                            pm                            2                                                    -                                                      C                            pm                            ′2                                                                          )                                            ⁢                                                                        D                          p                                                ⁡                                                  (                          t                          )                                                                    ⁢                                              E                        pm                        2                                            ⁢                                              E                        im                        2                                                                                                        +                                                1                  4                                ⁢                                  R                  2                                ⁢                                                      ∑                                          i                      ;                                              i                        ≠                        p                                                              K                                    ⁢                                                            ∑                                                                        j                          ;                                                      j                            ≠                            p                                                                          ,                        i                                            K                                        ⁢                                                                  ∑                        m                        M                                            ⁢                                                                        (                                                                                    C                              pm                              2                                                        -                                                          C                              pm                              ′2                                                                                )                                                ⁢                                                  E                          im                          2                                                ⁢                                                  E                          jm                          2                                                                                                                                +                              a                5                                      ≈                                                            eB                  ⁡                                      (                                                                  2                        ⁢                                                                              D                            p                                                    ⁡                                                      (                            t                            )                                                                                              +                      K                      -                      1                                        )                                                  ⁢                                  i                  data                                            +                                                B                                      4                    ⁢                    F                                                  ⁢                                                      D                    p                                    ⁡                                      (                    t                    )                                                  ⁢                                  (                                      K                    -                    1                                    )                                ⁢                                  i                  data                  2                                            +                                                                                          (                                              K                        -                        1                                            )                                        ⁢                                          (                                              K                        -                        2                                            )                                        ⁢                    B                                                        16                    ⁢                    F                                                  ⁢                                  i                  data                  2                                ⁢                                  α                  2                                            +                              a                5                                                    ⁢                                  ⁢                  where          ,                                    a              1                        =                                          i                s                2                            _                                ,                                    a              2                        =                                          i                b                2                            _                                ,                                    a              3                        =                                          i                                  s                  -                  b                                2                            _                                ,                                    a              4                        =                                          i                                  b                  -                  b                                2                            _                                ,                                    a              5                        =                                          i                c                2                            _                                                          Equation        ⁢                                  ⁢                  (          3          )                    
Where, Eim, fim and Φim mean electric field intensity, optical frequency and phase of optical frequency chip m of code i, respectively. i means integer from 1 to K (K is natural number of 2 or more), m means integer from 1 to M (M is natural number of 2 or more), Cpm and Cpm′ are light power transmission function of two outputs of the decoder 131 of the optical frequency chip m for code p, and F means frequency separation between the chips. Further, a1, a2, a3, a4 and a5 mean shot noise of code p that is a selection code being selected as a reception object, shot noise of non-selection codes of codes other than that, beat noise between selection code and non-selection code, beat noise between non-selection code and non-selection code, and receiver noise including dark current, respectively, and are assumed to be approximated as a Gaussian distribution in each variance. Further, e means an elementary electric charge, R means detector responsivity of the differential detectors 132a, 132b, B means bandwidth in the electrical domain of the receiver, and Dp(t) means data value of the code p at time t and its value is 0 or 1. Values corresponding to the code other than code p are shown by values averaged data values of 0 and 1. For simplicity, signal current intensity of all codes is considered to be identical, multiple access interference suppression ratio αi is considered to be identical value α, electric field intensity and polarization state of optical frequency chip constituting coded light of each code are considered identical, polarization state of the coded light with different code is assumed to exhibit uniform distribution, beat between chips with different numbers are assumed to lie outside receiver's bandwidth, and frequency difference between chips with the same number of coded light with different code is assumed to be uniformly distributed over half of the frequency separation F. Therefore, only B/F of beat noises in Equation (3) affects noises variance. In order to reduce influences of beat noise upon the number of multiplexed code in this conventional example for assessment purpose, it is assumed that the beat noise between non-selection code and non-selection code could be suppressed by multiple access interference suppression ratio α. The bit error rate (BER) in this example can be expressed by Equation (4). In Equation (4), erfc means complementary error function and idata means signal current intensity.
                    BER        =                                                             ⁢                                          ⁢                                    1              4                        ⁢                                                  ⁢                          erfc              (                                                          ⁢                                                1                                      2                    ⁢                                          2                                                                      ⁢                                                                  ⁢                                                      i                    data                                                                                                                                eB                          ⁡                                                      (                                                          K                              +                              1                                                        )                                                                          ⁢                                                  i                          data                                                                    +                                                                        (                                                                                                                                                                                                                                                                        (                                                                                  K                                          -                                          1                                                                                )                                                                            ⁢                                      B                                                                                                              4                                      ⁢                                      F                                                                                                        +                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              (                                                                                                      K                                                    -                                                    1                                                                                                    )                                                                                                                                                                                                                                                                                                                                                                                          (                                                                                                          K                                                      -                                                      2                                                                                                        )                                                                                                    ⁢                                                  B                                                  ⁢                                                                                                                                                                                                          ⁢                                                                                                      α                                                    2                                                                                                                                                                                                                                                                                                                            16                                            ⁢                                            F                                                                                                                          +                                                                                                                                                                                                                                                                                                                                                      (                                                                                          K                                              -                                              1                                                                                        )                                                                                    ⁢                                                                                      α                                            2                                                                                                                          2                                                                                                                                                                                                                                                          )                                                ⁢                                                  i                          data                          2                                                                    +                                                                        i                          c                          2                                                _                                                                                                        ⁢                                                          )                                                          Equation        ⁢                                  ⁢                  (          4          )                    
FIG. 2 shows a relationship between the number of multiplexed codes and power penalty. In FIG. 2, dotted line shows power penalty as a function of the number of multiplexed codes that follows Equation (4). Multiple acess interference suppression ratio is considered to be 30.7 dB. It is known from FIG. 2 that, as shown by dotted line, the penalty due to shot noise and beat noise from other coded light can not be neglected in the conventional example. In this case, one of methods for improving receiving sensitivity is to perform coherent detection using sufficient strong local light which is in a predetermined frequency relationship with coded light (for example, see Non-patent Document 2). According to the conventional art, as the method for applying coherent detection to OCDM, for example, Patent Document 1 is mentioned.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 10-013306
Non-patent Document 1: C. F. Lam, et al. “Experimental Demonstration of bipolar optical CDMA System Using a Balanced Transmitter and Complimentary Spectral Encoding”, IEEE Photon. Technol. Lett. Vol. 10, No. 10, pp. 1504 to 1506 (1998)
Non-patent Document 2: Coherent Optical Communication Engineering, The Ohmsha, Ltd.